This invention relates to display systems. More particularly, it is concerned with television-like display systems for producing characters of the dot matrix type.
Display systems of the type for displaying data from a computer on television monitors are well-known. These systems are generally similar and employ a central processing unit in which data processing tasks may be performed or in which data is stored. Data is communicated from the central processing unit selectively to several display stations or terminals through an interface unit. Typically information to be displayed at a display terminal is transmitted from the interface unit to a refresh memory of the display terminal. Each display terminal employs a standard closed circuit television monitor for displaying the data in its refresh memory and may also employ a keyboard to permit an operator at the display terminal to communicate with the central processing unit.
Display systems of this general type typically display information as alphanumeric characters. Digital code words representing alphanumeric characters to be displayed are loaded into storage positions in the refresh memory of a display terminal. Each storage position corresponds to a specific row and column character location on the display surface of the display device. The contents of the storage positions are repeatedly read out of the refresh memory in timed relationship with the sweeping of the raster scanline pattern over the display surface. Each code word read out of the refresh memory together with information identifying the particular line of the number of scanlines required to produce a character are applied to a character generator.
The character generator typically is a read only memory (ROM) which converts the coded information to bits of character image data. These bits are gated into the video stream to the display device where they are displayed as a series of dots forming one line of the image of the character. During each succeeding readout of the storage position during a frame of the raster scanline pattern the code word is again applied to the character generator together with the appropriate scanline information. The character generator produces bits of character image data which are displayed as another series of dots. Thus, the character represented by the code word is produced on the display surface in the location designated by its storage position in the refresh memory.
As is well understood in the display art the technique of using digital code words and ROM character generators greatly reduces the storage capacity required in each refresh memory and also in the central processing unit. Rather than store and transmit an image bit for each dot of a character to be displayed, only a code word of a relatively few bits representing a character is processed throughout the system until it is applied to the character generator. The character generator requires only the code word and information from the timing system on the particular scanline of data to be written in order to generate the video image bits for that scanline.
Systems of this general type have also been used for displaying graphical information in addition to alphanumeric text. Code words representing graphical characters are loaded into the refresh memory in the same manner a code words representing alphanumeric characters. The graphical code words are applied to ROM graphical character generators similar to the alphanumeric character generators to generate the video image bits for writing the graphical character. Systems for displaying both alphanumeric and graphical characters are described in U.S. Pat. No. 3,781,850 entitled "Television Type Display System for Displaying Information in the Form of Curves or Graphs" which issued Dec. 25, 1973, to Francis A. Gicca, Francis C. Passavant, and Allen J. Worters, and U.S. Pat. No. 3,778,811 entitled "Display System with Graphic Fill-In" which issued Dec. 11, 1973, to Francis A. Gicca, Francis C. Passavant, William W. Stebbins, and Alan J. Worters.
Under certain conditions it is necessary to display information other than in the form of standard alphanumeric and graphical characters. For example, it may be necessary to display the characters of an alphabet other than the Latin alphabet or to display pictorial or schematic symbols. Known types of display systems employing refresh memories and ROM character generators, however, are limited to displaying only the standard characters provided by the code word format and the character generators employed in the system.